A semiconductor memory array includes a plurality of unit memory cells that are electrically connected to one another. In dynamic random access memory (DRAM), for example, a unit memory cell may include one switch and one capacitor. A DRAM has high integration density and high operation speed. However, when power is not supplied to the DRAM, data stored in the DRAM is erased. An example of non-volatile memory is flash memory, in which stored data is not erased when power is not supplied. Although flash memory has non-volatile characteristics, flash memory has low integration density and low operation speed as compared to DRAM.
Resistive random access memory (RRAM) is one of the non-volatile memory devices. An RRAM is a resistive type memory based on a characteristic that a resistance of a transition metal oxide varies according to a voltage applied thereto, and the resistance is used to store a bit of data in an RRAM cell instead of an electronic charge used in the DRAM. An RRAM consists of a metal-insulator-metal (MIM) structure in which the insulating materials show a resistive switching behavior.
Conductive paths or so-called conductive filaments (CF) are formed in such insulating materials after an electroforming step. After the generation of the CF, when passing a current through the capacitor like structure, a low resistance state (LRS) is identified, indicating a digital signal “0” or “1”. The RRAM can then be RESET by receiving a voltage high enough to break the CF in the insulating materials, wherein a high resistance state (HRS) is identified, indicating a digital signal “1” or “0”. The two states (LRS, HRS) of the RRAM are reversible based on a voltage applied thereto.